In this first lesson we will go over what voltage, current and resistance is and how they work together in Ohm’s law.
But first… Electrons and currentflow
Electricity is all about moving electrons. They do all the work in electronic applications. Electrons create charge. Electrical engineering is all about manipulating these charges.
Most powersources have two terminals. One of them is called the negative (-) terminal and the other the positive (+) terminal. For electrons to flow, there must be a closed circuit. This is done by connecting the two terminals with a conductor. A conductor is a material that is good at transporting electrons like copper. A material that is very bad at transporting the electrons is called an insulator like air or plastic.
When working with electronics and reading schematics its important to know that the flow of current in which we work goes from positive (+) to negative (-). This is called conventional current flow. The electronflow goes the exact other way. Why do we not use electronflow instead of conventional current flow? Well… There were mistakes made in history when determining the actual flow of electrons. So remember: We use conventional current flow in which the charge flows from a positive terminal to a negative terminal.
But before charge can flow through a conductor from the positive (+) to negative (-) terminal, there has to be a difference in charge between these terminals to actually make one more positive than the other. This is called voltage (U): The difference in charge between two points. Its important to know that voltage is always measured between two points. Voltage is measured in volts.
The rate in which charge flows is called current (I) and is measured in ampères. Higher ampère or ‘amps’ means more charge travels through the conductor in a certain amount of time. How can we control this flowrate?
The conductivity of a material is determined by the resistance of the material. We talked about how copper is more conductive than air because the electrons can easely pass through it. The harder it is for a charge to flow, the less conductive the medium is in which it travels. When the electrons have a hard time going through the material, the rate at which it travels goes down. When the rate goes down, the amps go down. See where we are going to? By introducing resistance it is possible to influence the amps going through a circuit. Resistance is measured in Ohm (R).
The equation that will start it all for starting electrical engineers is called Ohm’s law. It shows the relationship between voltage (U), current (I) and resistance (R)
R = U / I
Let’s look at our first schematic:
On top we see a voltage of 5 volts. We know that voltage is the difference between two points. What point is the second point? In this schematic that’s GND or ground. Appearantly there is a voltage of 5 volts over the square box thing. That box thing is a resistor. Resistors in real life come in all kinds of values.
Let’s use the values in the schematic to find out how many amps go through the resistor!
R = 100 Ω ( U = 5 V
I = ? A
When doing a bit of algebra we can arrange Ohm’s law to calculate the amount of current:
R= U / I => I = U / R = 5 / 100 = 0.05 A
Appearantly the currentflow is 50 milliampere. So what happens if we take a bigger resistance? Let’s look at our next schematic:
When comparing this circuit with the previous one, you will notice that the value of the resistor got 10 times bigger. Let’s do the math again!
R = 1000 Ω
U = 5 V
I = ? A
I = U / R = 5 / 1000 = 0.005 A
Looks like when we made the resistor 10 times bigger, the current became 10 times smaller! Now we’ve proven that a larger resistance limits the currentflow in a circuit!
How else could we limit the current flow? Use Ohm’s law and the schematics to find out yourself!